The present invention relates to an endoscopic photographing apparatus using a solid state image sensor.
A TV camera having a solid state image sensor has been used to observe or photograph an endoscopic body cavity image. An object to be examined is illuminated by an observation lamp such as a halogen lamp in order to observe it for immediate diagnosis. However, the object must be illuminated by a strobe light source such as a xenon lamp in order to photograph it to obtain a still image for record or later diagnosis. In this manner, the light sources are switched in accordance with the observation mode and the still image photographing mode.
In the endoscopic photographing apparatus, the TV camera performs interlaced scanning at a speed of 1/60 sec (about 16.7 msec (millisecond)). The strobe light source flashes within a range between several hundred microseconds and 10 msec in the automatic flash mode. When the strobe light source flashes for a maximum period of 10 msec, this period comes close to the scanning time (16.7 msec). When the flashing start moment of the strobe light source considerably deviates from the scanning start moment of one field, the light rays from the strobe light source cannot be effectively utilized. Furthermore, when the diagnostic light source (observation mode) is switched to the strobe light source (still image photographing mode), light rays from these two light sources are mixed, resulting in a foggy image. As a result, a clear image cannot be obtained. In addition to this disadvantage, when the strobe flashing period is set to be shortest (e.g., when the strobe light source flashes for 1/1000 sec or less), 90% or more of the exposure time (1/60 sec) becomes an idle time which cannot be used for charge storage. During this nonstorage time, the stored charge distribution changes due to a dark current. As a result, a good video image cannot be obtained.